s390/ccwgroup: use bus->dev_groups for bus-based sysfs attributes

Bus drivers have their own way of describing the sysfs attributes that
all devices on a bus should provide.
Switch ccwgroup_attr_groups over to use bus->dev_groups, and thus
free up dev->groups for usage by the ccwgroup device drivers.

While adjusting the attribute naming, use ATTRIBUTE_GROUPS() to get rid
of some boilerplate code.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>